1. Field of the Invention
The present invention relates, in general, to a valve timing control device for an internal combustion engine, and in particular, to a valve timing control device comprising a variable valve timing mechanism for controlling the valve overlap period.
2. Description of the Related Art
In an internal combustion engine, when the valve overlap period, during which the intake valve and the exhaust valve are opened simultaneously at the end of an exhaust stroke, is made long, the trapping efficiency and the scavenging efficiency increase so that good engine performance can be obtained. On the other hand, in engine operating conditions under which the degree of opening of the throttle valve is relatively small so that the negative pressure in an intake port becomes high, once the valve overlap period is made long, the amount of back-flow exhaust gas into the intake port becomes large so that combustion deteriorates. Accordingly, it is desirable that the valve overlap period is varied in accordance with the engine operating condition. For this purpose, a variable valve timing mechanism capable of varying the valve overlap period has already been suggested.
According to the usual valve timing control device for controlling the valve overlap period by means of the variable valve timing mechanism, during engine operating conditions under which the degree of opening the throttle valve is relatively small, the valve overlap period is made short to prevent deterioration of combustion caused by a large amount of back-flow exhaust gas. However, Japanese Unexamined Patent Publication Nos. 59-103910 and 2-298614 disclose a valve timing control device which makes the valve overlap period long during an idle condition when the engine has not warmed up. This is intended to positively utilize the back-flow of exhaust gas, as exhaust gas recirculation, by leading a portion of exhaust gas into the intake port to reduce the amount of NOx in the exhaust gas, by lowering the combustion temperature, and by heating the intake port to raise the intake air temperature.
However, during an engine idle speed (i.e. the idle condition) condition when the engine has not warmed up, combustion is very unstable so that the engine can stall when the amount of back-flow exhaust gas is increased. The valve timing control device disclosed in JP No. 2-298614 mentioned above increases the amount of fuel and delays the ignition time to improve the combustion at this time. However, these cannot improve the combustion sufficiently. As a result, engine stall can still occur.